Method and apparatus of monitoring pdcch in wireless communication system

ABSTRACT

A method and an apparatus of monitoring a physical downlink control channel (PDCCH) in a wireless communication system, carried in a user equipment (UE), are provided. The method includes receiving a PDCCH map, and monitoring a set of PDCCH candidates based on the PDCCH map.

TECHNICAL FIELD

The present invention relates to wireless communications, and moreparticularly, to a method and an apparatus of monitoring a physicaldownlink control channel (PDCCH) in a wireless communication system.

BACKGROUND ART

In wireless communication systems, one base station (BS) generallyprovides services to a plurality of user equipments (UEs). The BSschedules user data for the plurality of UEs, and transmits the userdata together with control information containing scheduling informationfor the user data. In general, a channel for carrying the controlinformation is referred to as a control channel, and a channel forcarrying the user data is referred to as a data channel. The UE findscontrol information of the UE by searching for the control channel, andprocesses data of the UE by using the control information.

In order for the UE to receive user data assigned to the UE, controlinformation for the user data on a control channel must be received. Ina given bandwidth, a plurality of pieces of control information for aplurality of UEs are generally multiplexed within one transmissioninterval. That is, to provide a service to the plurality of UEs, the BSmultiplexes the plurality of pieces of control information for theplurality of UEs and then transmits the control information through aplurality of control channels. The UE searches for control channel ofthe UE among the plurality of control channels.

Blind decoding is one of schemes for detecting specific controlinformation from the plurality of pieces of multiplexed controlinformation. The blind decoding attempts to recover a control channel byusing several combinations of information in a state where a UE has noinformation required to recover the control channel. That is, in a statewhere the UE does not know whether control information transmitted fromthe BS is control information of the UE and the UE does not know inwhich portion the control information of the UE exists, the UE decodesall pieces of given control information until the control information ofthe UE is found. The UE can use information unique to each UE to detectthe control information of the UE. For example, when the BS multiplexescontrol information of each UE, an identifier unique to each UE can betransmitted by being masked onto a cyclic redundancy check (CRC). TheCRC is a code used for error detection. The UE de-masks uniqueidentifier of the UE from the CRC of the received control information,and then can detect the control information of the UE by performing CRCchecking.

Meanwhile, as a mobile communication system of a next generation (i.e.,post-3rd generation), an international mobile telecommunication-advanced(IMT-A) system is standardized aiming at support of an Internet protocol(IP)-based seamless multimedia service in an internationaltelecommunication union (ITU) by providing a high-speed transmissionrate of 1 gigabits per second (Gbps) in downlink communication and 500megabits per second (Mbps) in uplink communication. In a 3rd generationpartnership project (3GPP), a 3GPP long term evolution-advanced (LTE-A)system is considered as a candidate technique for the IMT-A system. TheLTE-A system is evolved to increase a completion level of the LTEsystem, and is expected to maintain backward compatibility with the LTEsystem. This is because the provisioning of compatibility between theLTE-A system and the LTE system is advantageous in terms of userconvenience, and is also advantageous for a service provider sinceexisting equipment can be reused.

In general, a wireless communication system is a single carrier systemsupporting a single carrier. The transmission rate is proportional totransmission bandwidth. Therefore, for supporting a high-speedtransmission rate, transmission bandwidth shall be increased. However,except for some areas of the world, it is difficult to allocatefrequencies of wide bandwidths. For effectively using fragmented smallfrequency bands, a spectrum aggregation (also referred to as bandwidthaggregation or carrier aggregation) technique is being developed. Thespectrum aggregation technique is to obtain the same effect as if whicha frequency band of a logically wide bandwidth may be used byaggregating a plurality of physically discontiguous frequency bands in afrequency domain. Through the spectrum aggregation technique, multiplecarrier (multi-carrier) can be supported in the wireless communicationsystem. The wireless communication system supporting multi-carrier isreferred to as a multi-carrier system. The carrier may be also referredto as a radio frequency (RF), component carrier (CC), etc.

However, if a BS transmits a control channel and a UE monitors thecontrol channel with a same manner used in a single carrier system in amulti-carrier system, the complexity of blind decoding is significantlyincreased.

Accordingly, there is a need for a method and an apparatus ofeffectively transmitting a control channel and monitoring a controlchannel in a multi-carrier system.

DISCLOSURE OF INVENTION Technical Problem

The present invention provides a method and an apparatus of monitoring aphysical downlink control channel (PDCCH) in a wireless communicationsystem.

Technical Solution

In an aspect, a method of monitoring a physical downlink control channel(PDCCH) in a wireless communication system, carried in a user equipment(UE), is provided. The method includes receiving a PDCCH map from a basestation (BS), and monitoring a set of PDCCH candidates based on thePDCCH map.

Preferably, the PDCCH map comprises a monitoring set field, themonitoring set field indicating N downlink component carriers (CCs) outof L downlink CCs (L≧N, where L and N each is a natural number), and aUE monitors the set of PDCCH candidates in each of the N downlink CC.

Preferably, the PDCCH map is received on a PDCCH.

Preferably, the PDCCH map is received through a constant downlink CC outof a plurality of downlink CCs.

Preferably, the PDCCH map is received through a downlink CC, thedownlink CC is hopped among a plurality of downlink CCs in accordancewith a hopping rule.

Preferably, the PDCCH map is received via radio resource control (RRC)signal.

Preferably, the PDCCH map comprises a control channel element (CCE)field, the CCE field indicating Y CCE aggregation levels out of X CCEaggregation levels (X≧Y, where X and Y each is a natural number), and aUE monitors the set of PDCCH candidates at each of the Y CCE aggregationlevels.

Preferably, the PDCCH map comprises a monitoring set field and a CCEfield.

In another aspect, a UE is provided. The UE includes a radio frequency(RF) unit transmitting and/or receiving a radio signal and a processorcoupled with the RF unit and configured to receive a PDCCH map, andmonitor a set of PDCCH candidates based on the PDCCH map.

In still another aspect, a method of transmitting a PDCCH in a wirelesscommunication system, carried in a BS, is provided. The method includestransmitting a PDCCH map to a UE, and transmitting a PDCCH in accordancewith the PDCCH map to the UE.

Advantageous Effects

A method and an apparatus of effectively monitoring a physical downlinkcontrol channel (PDCCH) are provided. Accordingly, overall systemperformance can be improved.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing a wireless communication system.

FIG. 2 shows an example of a plurality of component carriers (CCs) usedin a multi-carrier system.

FIG. 3 is a block diagram showing an example of a multi-carrier system.

FIG. 4 shows an example of a plurality of physical channels (PHYs).

FIG. 5 shows an example of a bandwidth used by a PHY.

FIG. 6 shows an example of an asymmetric structure of downlink anduplink in a multi-carrier system.

FIG. 7 shows a structure of a radio frame.

FIG. 8 shows an example of a resource grid for one downlink slot.

FIG. 9 shows a structure of a radio frame and a subframe in a frequencydivision duplex (FDD) system.

FIG. 10 shows an example of an resource element group (REG) structurewhen a base station (BS) uses one or two transmit (Tx) antennas.

FIG. 11 shows an example of an REG structure when a BS uses four Txantennas.

FIG. 12 shows an example of mapping of a physical control formatindicator channel (PCFICH) to REGs.

FIG. 13 is a flow diagram showing an example of a method of transmittingdata and receiving data performed by a user equipment (UE).

FIG. 14 is a flowchart showing an example of a method of configuring aphysical downlink control channel (PDCCH).

FIG. 15 shows an example of a method of multiplexing a plurality ofPDCCHs for a plurality of UEs, performed by a BS.

FIG. 16 shows an example of a method of monitoring a control channel,performed by a UE.

FIG. 17 shows an example of a PDCCH transmission method in amulti-carrier system.

FIG. 18 is a flow diagram showing a control channel transmission methodand/or a control channel monitoring method according to an embodiment ofthe present invention.

FIG. 19 shows an example of transmitting a PDCCH by using a PDCCH map ina multi-carrier system.

FIG. 20 shows another example of transmitting a PDCCH by using a PDCCHmap in a multi-carrier system.

FIG. 21 shows an example of semi-statically configured a PDCCH map.

FIG. 22 shows another example of semi-statically configured a PDCCH map.

FIG. 23 shows a control channel monitoring method performed by a UE in amulti-carrier system.

FIG. 24 is a block diagram showing wireless communication system toimplement an embodiment of the present invention.

MODE FOR THE INVENTION

FIG. 1 is a block diagram showing a wireless communication system.

Referring to FIG. 1, a wireless communication system 10 includes atleast one base station (BS) 11. The BSs 11 provide communicationservices to specific geographical regions (generally referred to ascells) 15 a, 15 b, and 15 c. The cell can be divided into a plurality ofregions (referred to as sectors). A user equipment (UE) 12 may be fixedor mobile, and may be referred to as another terminology, such as amobile station (MS), a user terminal (UT), a subscriber station (SS), awireless device, a personal digital assistant (PDA), a wireless modem, ahandheld device, etc. The BS 11 is generally a fixed station thatcommunicates with the UE 12 and may be referred to as anotherterminology, such as an evolved node-B (eNB), a base transceiver system(BTS), an access point, etc.

Hereinafter, a downlink (DL) denotes communication from the BS to theUE, and an uplink (UL) denotes communication from the UE to the BS. Inthe DL, a transmitter may be a part of the BS, and a receiver may be apart of the UE. In the UL, the transmitter may be a part of the UE, andthe receiver may be a part of the BS.

The wireless communication system supports multi-antenna. Thetransmitter may use a plurality of transmit (Tx) antennas, and thereceiver may use a plurality of receive (Rx) antennas. The Tx antenna isa logical or physical antenna used to transmit one signal or one stream,and the Rx antenna is a logical or physical antenna used to receive onesignal or one stream.

If the transmitter and the receiver use multi-antenna, the wirelesscommunication system may be called as multiple input multiple output(MIMO) system.

FIG. 2 shows an example of a plurality of component carriers (CCs) usedin a multi-carrier system.

Referring to FIG. 2, a multi-carrier system may use N CCs (CC #1, CC #2,. . . , CC #N). Although it is described herein that adjacent CCs arephysically discontiguous in a frequency domain, this is for exemplarypurposes only. Adjacent CCs may be physically contiguous in a frequencydomain

Therefore, a frequency band of a logically wide bandwidth may be used inthe multi-carrier system by aggregating a plurality of physicallydiscontiguous and/or contiguous CCs in a frequency domain.

In downlink, a BS concurrently can transmit information to one UEthrough one or more CCs. In uplink, the UE can also transmit data to theBS through one or more CCs.

FIG. 3 is a block diagram showing an example of a multi-carrier system.

Referring to FIG. 3, each of a transmitter 100 and a receiver 200 uses NCCs (CC #1, CC #2, . . . , CC #N) in a multi-carrier system. A CCincludes one or more physical channels (hereinafter, simply referred toas PHYs). A wireless channel is established between the transmitter 100and the receiver 200.

The transmitter 100 includes a plurality of PHYs 110-1, . . . , 110-M, amulti-carrier multiplexer 120, and a plurality of Tx antennas 190-1, . .. , 190-Nt. The receiver 200 includes a multi-carrier demultiplexer 210,a plurality of PHYs 220-1, . . . , 220-L, and a plurality of Rx antennas290-1, . . . , 290-Nr. The number M of PHYs of the transmitter 100 maybe identical to or different from the number L of PHYs of the receiver200. Although it is described herein that each of the transmitter 100and the receiver 200 includes a plurality of antennas, this is forexemplary purposes only. The transmitter 100 and/or the receiver 200includes a single antenna.

The transmitter 100 generates Tx signals from information based on the NCCs, and the Tx signals are transmitted on M PHYs 110-1, . . . , 110-M.The multi-carrier multiplexer 120 combines the Tx signals so that the Txsignals can be simultaneously transmitted on the M PHYs. The combined Txsignals are transmitted through the Nt Tx antennas 190-1, . . . ,190-Nt. The Tx radio signals are received through the Nr Rx antennas290-1, . . . , 290-Nr of the receiver 200 through the wireless channel.The Rx signals are de-multiplexed by the multi-carrier demultiplexer 210so that the Rx signals are separated into the L PHYs 220-1, . . . ,220-L. Each of the PHYs 220-1, . . . , 220-L recovers the information.

The multi-carrier system may include one or more carrier modules. Thecarrier module upconverts a baseband signal to a carrier frequency to bemodulated onto a radio signal, or downconverts a radio signal to recovera baseband signal. The carrier frequency is also referred to as a centerfrequency. The multi-carrier system may use a plurality of carriermodules for each carrier frequency, or use a carrier module which canchange a carrier frequency.

FIG. 4 shows an example of a plurality of PHYs. FIG. 4 shows an exampleof N CCs consisting of M PHYs (PHY #1, PHY #2, . . . , PHY #M).

Referring to FIG. 4, each of M PHYs has a specific bandwidth (BW). AnPHY #m has a center frequency f_(c,m) and a bandwidth ofN_(IFFT,m)×Δf_(m) (where m=1, . . . , M). Herein, N_(IFFT,m) denotes aninverse fast Fourier transform (IFFT) size of the PHY #m, and M_(m)denotes a subcarrier spacing of the PHY #m. The IFFT size and thesubcarrier spacing may be different or identical for each PHY. Centerfrequencies of the respective PHYs may be arranged with a regularinterval or an irregular interval.

According to a UE or a cell, each PHY may use a bandwidth narrower thana maximum bandwidth. For example, if it is assumed that each PHY has amaximum bandwidth of 20 mega Hertz (MHz), and M is 5, then a fullbandwidth of up to 100 MHz can be supported.

FIG. 5 shows an example of a bandwidth used by a PHY.

Referring to FIG. 5, if it is assumed that a maximum bandwidth of thePHY is 20 MHz, the PHY can use a bandwidth (e.g., 10 MHz, 5 MHz, 2.5MHz, or 1.25 MHz) narrower than the maximum bandwidth. Regardless of abandwidth size used by the PHY in downlink, a synchronization channel(SCH) may exist in each PHY. The SCH is a channel for cell search. Thecell search is a procedure by which a UE acquires time synchronizationand frequency synchronization with a cell and detects a cell identifier(ID) of the cell. If the SCH is located in all downlink PHYs, all UEscan be synchronized with the cell. In addition, if a plurality ofdownlink PHYs are allocated to the UE, cell search may be performed foreach PHY or may be performed only for a specific PHY.

As such, a UE or a BS can transmit and/or receive information based onone or more PHYs in the multi-carrier system. The number of PHYs used bythe UE may be different from or equal to the number of PHYs used by theBS. In general, the BS can use M PHYs, and the UE can use L PHYs (M≧L,where M and L are natural numbers). Herein, L may differ depending on atype of the UE.

The multi carrier system can have several types of uplink and downlinkconfigurations. In a frequency division duplex (FDD) system or a timedivision duplex (TDD) system, a structure of downlink and uplink may bean asymmetric structure in which an uplink bandwidth and a downlinkbandwidth are different from each other. Alternatively, the structure ofdownlink and uplink may be configured in which an uplink bandwidth and adownlink bandwidth are identical to each other. In this case, thestructure of downlink and uplink may be configured to a symmetricstructure in which the same number of PHYs exist in both uplink anddownlink transmissions or an asymmetric structure in which the number ofPHY differs between uplink and downlink transmissions.

FIG. 6 shows an example of an asymmetric structure of downlink anduplink in a multi-carrier system. A transmission time interval (TTI) isa scheduling unit for information transmission. In each of the FDDsystem and the TDD system, a structure of downlink and uplink is anasymmetric structure. If the structure of downlink and uplink is anasymmetric structure, a specific link may have a higher informationthroughput. Therefore, system can be optimized flexibly.

Hereinafter, for convenience of explanation, it is assumed that a CCincludes one PHY.

All transmission/reception methods used in a single carrier system usingcan also be applied to each CC of the transmitter and the receiver in amulti-carrier system. In addition, it is desirable for the multi-carriersystem to maintain backward compatibility with the single carrier systemwhich is legacy system of the multi-carrier system. This is because theprovisioning of compatibility between the multi-carrier system and thesingle carrier system is advantageous in terms of user convenience, andis also advantageous for a service provider since existing equipment canbe reused.

Now, a single carrier system will be described.

FIG. 7 shows a structure of a radio frame.

Referring to FIG. 7, the radio frame consists of 10 subframes. Onesubframe consists of two slots. Slots included in the radio frame arenumbered with slot numbers #0 to #19. A time required to transmit onesubframe is defined as a TTI. For example, one radio frame may have alength of 10 milliseconds (ms), one subframe may have a length of 1 ms,and one slot may have a length of 0.5 ms.

The structure of the radio frame is for exemplary purposes only, andthus the number of subframes included in the radio frame or the numberof slots included in the subframe may change variously.

FIG. 8 shows an example of a resource grid for one downlink slot.

Referring to FIG. 8, the downlink slot includes a plurality oforthogonal frequency division multiplexing (OFDM) symbols in a timedomain and NDL resource blocks (RBs) in a frequency domain. The OFDMsymbol is for expressing one symbol period, and may be referred to as anorthogonal frequency division multiple access (OFDMA) symbol or a singlecarrier-frequency division multiple access (SC-FDMA) symbol according toa multiple access scheme. The number NDL of resource blocks included inthe downlink slot depends on a downlink transmission bandwidthconfigured in a cell. One RB includes a plurality of subcarriers in thefrequency domain.

Each element on the resource grid is referred to as a resource element(RE). Although it is described herein that one RB includes 7×12 resourceelements consisting of 7 OFDM symbols in the time domain and 12subcarriers in the frequency domain for example, the number of OFDMsymbols and the number of subcarriers in the RB are not limited thereto.Thus, the number of OFDM symbols and the number of subcarriers maychange variously depending on a cyclic prefix (CP) length, a subcarrierspacing, etc. For example, when using a normal CP, the number of OFDMsymbols is 7, and when using an extended CP, the number of OFDM symbolsis 6.

The resource grid for one downlink slot of FIG. 8 can be applied to aresource grid for an uplink slot.

FIG. 9 shows a structure of a radio frame and a subframe in a FDDsystem.

Referring to FIG. 9, the radio frame includes 10 subframes, and eachsubframe includes two consecutive slots. When using a normal CP, thesubframe includes 14 OFDM symbols. When using an extended CP, thesubframe includes 12 OFDM symbols. A SCH is transmitted in every radioframe. The SCH includes a primary (P)-SCH and a secondary (S)-SCH. TheP-SCH is transmitted through a last OFDM symbol of a 1st slot of asubframe 0 and a subframe 5 in a radio frame. When using the normal CP,the P-SCH is an OFDM symbol 6 in the subframe, and when using theextended CP, the P-SCH is an OFDM symbol 5 in the subframe. The S-SCH istransmitted through an OFDM symbol located immediately before an OFDMsymbol on which the P-SCH is transmitted.

A maximum of three OFDM symbols (i.e., OFDM symbols 0, 1, and 2) locatedin a front portion of a 1st slot in every subframe correspond to acontrol region. The remaining OFDM symbols correspond to a data region.A physical downlink shared channel (PDSCH) can be assigned to the dataregion. Downlink data is transmitted on PDSCH.

Cntrol channels such as a physical control format indicator channel(PCFICH), a physical HARQ (hybrid automatic repeat request) indicatorchannel (PHICH), a physical downlink control channel (PDCCH) etc., canbe assigned to the control region.

Resource element groups (REGs) are used for defining the mapping of acontrol channel to resource elements.

FIG. 10 shows an example of an REG structure when a BS uses one or twoTx antennas. FIG. 11 shows an example of an REG structure when a BS usesfour Tx antennas. In FIGS. 10 and 11, it is assumed that a maximum ofthree OFDM symbols (i.e., OFDM symbols 0, 1, and 2) located in a frontportion of a 1st slot in a subframe are control regions.

Referring to FIGS. 10 and 11, Rp indicates a resource element which isused to transmit a reference signal (hereinafter referred to as an ‘RS’)through antenna p (pε{0, 1, 2, 3}). The RS may be also referred to as apilot. One REG is composed of four adjacent resource elements in thefrequency domain other than resource elements which are used for RStransmission. In the OFDM symbol 0 in the subframe, two REGs existwithin one resource block in the frequency domain. It is to be notedthat the above REG structures are only illustrative and the number ofresource elements included in the REG may change in various ways.

The PHICH carries an HARQ acknowledgement (ACK)/not-acknowledgement(NACK) for uplink data.

The PCFICH carries information about the number of OFDM symbols used fortransmission of PDCCHs in a subframe. Although the control regionincludes three OFDM symbols herein, this is for exemplary purposes only.According to an amount of control information, the PDCCH is transmittedthrough the OFDM symbol 0, or the OFDM symbols 0 and 1, or the OFDMsymbols 0 to 2. The number of OFDM symbols used for PDCCH transmissionmay change in every subframe. The PCFICH is transmitted through a 1stOFDM symbol (i.e., the OFDM symbol 0) in every subframe. The PCFICH canbe transmitted through a single antenna or can be transmitted through amulti-antenna using a transmit diversity scheme. When a subframe isreceived, the UE evaluates control information transmitted through thePCFICH, and then receives control information transmitted through thePDCCH.

The control information transmitted through the PCFICH is referred to asa control format indicator (CFI). For example, the CFI may have a valueof 1, 2, or 3. The CFI value may represent the number of OFDM symbolsused for PDCCH transmission in a subframe. That is, if the CIF value is2, the number of OFDM symbols used for PDCCH transmission in a subframeis 2. This is for exemplary purposes only, and thus informationindicated by the CFI may be defined differently according to a downlinktransmission bandwidth. For example, if the downlink transmissionbandwidth is less than a specific threshold value, the CFI values of 1,2, and 3 may indicate that the number of OFDM symbols used for PDCCHtransmission in the subframe is 2, 3, and 4, respectively.

The following table shows an example of a CFI and a 32-bit CFI codewordwhich generates by performing channel coding to the CFI.

TABLE 1 CFI codeword CFI <b₀, b₁, . . ., b₃₁> 1 <0, 1, 1, 0, 1, 1, 0, 1,1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1>2 <1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0,1, 1, 0, 1, 1, 0, 1, 1, 0> 3 <1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1,0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1, 0, 1, 1> 4 <0, 0, 0, 0, 0,0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, (Reserved) 0, 0, 0,0, 0, 0, 0, 0, 0, 0>

The 32-bit CFI codeword can be mapped to a 16 modulated symbols using aquadrature phase shift keying (QPSK) scheme. In this case, 16 resourceelements (or subcarriers) are used in PCFICH transmission. That is, 4REGs are used in PCFICH transmission.

FIG. 12 shows an example of mapping of a PCFICH to REGs.

Referring to FIG. 12, the PCFICH is mapped to 4 REGs, and the respectiveREGs to which the PCFICH are mapped are spaced apart from one another.An REG to which the PCFICH is mapped may vary according to the number ofresource blocks in a frequency domain. In order to avoid inter-cellinterference of the PCFICH, the REGs to which the PCFICH is mapped maybe shifted in a frequency domain according to a cell ID.

Now, a PDCCH will be described.

A control region consists of a set of control channel elements (CCEs).The CCEs are indexed 0 to N(CCE)-1, where N(CCE) is the total number ofCCEs constituting the set of CCEs in a downlink subframe. The CCEcorresponds to a plurality of REGs. For example, one CCE may correspondto 9 REGs. A PDCCH is transmitted on an aggregation of one or severalconsecutive CCEs. A PDCCH format and the possible number of bits of thePDCCH are determined according to the number of CCEs constituting theCCE aggregation. Hereinafter, the number of CCEs constituting the CCEaggregation used for PDCCH transmission is referred to as a CCEaggregation level. In addition, the CCE aggregation level is a CCE unitfor searching for the PDCCH. A size of the CCE aggregation level isdefined by the number of contiguous CCEs. For example, the CCEaggregation level may be an element of {1, 2, 4, 8}.

The following table shows an example of the PDCCH format, the number ofREGs and the number of PDCCH bits.

TABLE 2 PDCCH CCE Number Number format aggregation level of REGs ofPDCCH bits 0 1 9 72 1 2 18 144 2 4 36 288 3 8 72 576

Control information transmitted on the PDCCH is referred to as downlinkcontrol information (DCI). The DCI transports uplink schedulinginformation, downlink scheduling information, or an uplink power controlcommand, etc. The downlink scheduling information is also referred to asa downlink grant, and the uplink scheduling information is also referredto as an uplink grant.

FIG. 13 is a flow diagram showing an example of a method of transmittingdata and receiving data performed by a UE.

Referring to FIG. 13, a BS transmits an uplink grant to a UE at stepS11. The UE transmits uplink data to the BS based on the uplink grant atstep S12. The uplink grant may be transmitted on a PDCCH, and the uplinkdata may be transmitted on a physical uplink shared channel (PUSCH). Arelationship between a subframe in which a PDCCH is transmitted and asubframe in which a PUSCH is transmitted may be previously set betweenthe BS and the UE. For example, if the PDCCH is transmitted in an nthsubframe in a FDD system, the PUSCH may be transmitted in an (n+4)thsubframe.

The BS transmits an downlink grant to the UE at step S13. The UEreceives downlink data from the BS based on the downlink grant at stepS14. The downlink grant may be transmitted on a PDCCH, and the downlinkdata may be transmitted on a PDSCH. For example, the PDCCH and the PDSCHare transmitted in the same subframe.

As described above, a UE shall receive DCI on PDCCH to receive downlinkdata from a BS or transmit uplink data to a BS.

DCI may use a different DCI format in accordance with usage. Forexample, a DCI format for an uplink grant and a DCI format for adownlink grant is different each other. A size and usage of DCI maydiffer according to a DCI format.

The following table shows an example of the DCI format.

TABLE 3 DCI format Objectives 0 Scheduling of PUSCH 1 Scheduling of onePDSCH codeword 1A Compact scheduling of one PDSCH codeword 1BClosed-loop single-rank transmission 1C Paging, RACH response anddynamic BCCH 1D MU-MIMO 2 Scheduling of closed-loop rank-adapted spatialmultiplexing mode 2A Scheduling of open-loop rank-adapted spatialmultiplexing mode 3 TPC commands for PUCCH and PUSCH with 2 bit poweradjustments 3A TPC commands for PUCCH and PUSCH with single bit poweradjustments

Referring to above table, a DCI format 0 is used for PUSCH scheduling.The DCI format 0 is used for an uplink grant.

A DCI format 1 is used for scheduling of one PDSCH codeword. A DCIformat 1A is used for compact scheduling of one PDSCH codeword. A DCIformat 1B is used for compact scheduling of one PDSCH codeword in aclosed-loop rank 1 transmission mode. A DCI format 1C is used forpaging, random access channel (RACH) response, and dynamic broadcastcontrol channel (BCCH). A DCI format 1D is used for PDSCH scheduling ina multi-user (MU)-MIMO mode. A DCI format 2 is used for PDSCH schedulingin a closed-loop rank-adapted spatial multiplexing mode. A DCI format 2Ais used for PDSCH scheduling in an open-loop rank-adapted spatialmultiplexing mode. Each of from DCI format 1 to DCI format 2A is usedfor a downlink grant. However, the DCI format may differ according to ausage of DCI or transmission mode of a BS.

DCI formats 3 and 3A are used for transmission of a transmission powercontrol (TPC) command for a physical uplink control channel (PUCCH) anda PUSCH. The DCI formats 3 and 3A is used for an uplink power controlcommand.

Each DCI format consists of a plurality of information fields. The typeof information fields constituting a DCI format, the size of each of theinformation fields, etc. may differ according to the DCI format. Forexample, a downlink grant (or an uplink grant) includes resourceallocation field indicating radio resource. The downlink grant (or theuplink grant) may include further a modulation and coding scheme (MCS)field indicating modulation scheme and channel coding sheme. Inaddition, the downlink grant (or the uplink grant) may include furthervarious information fields.

FIG. 14 is a flowchart showing an example of a method of configuring aPDCCH.

Referring to FIG. 14, in step S21, a BS generates information bit streamin accordance with a DCI format. In step S22, the BS attaches a cyclicredundancy check (CRC) for error detection to the information bitstream. The information bit stream may be used to calculate the CRC. TheCRC is parity bits, and the CRC may be attached in front of theinformation bit stream or at the back of the information bit stream.

The CRC is masked with an identifier (referred to as a radio networktemporary identifier (RNTI)) according to an owner or usage of the DCI.The masking may be scrambling of the CRC with the identifier. Themasking may be an modulo 2 operation or exclusive or (XOR) operationbetween the CRC and the identifier.

If the DCI is for a specific UE, a unique identifier (e.g., cell-RNTI(C-RNTI)) of the UE may be masked onto the CRC. The C-RNTI may be alsoreferred to as a UE ID. The CRC can be masked with different RNTI exceptC-RNTI, such as paging-RNTI (P-RNTI) for a paging message, systeminformation-RNTI (SI-RNTI) for system information, random access-RNTI(RA-RNTI) for indicating random access response which is response ofrandom access preamble transmitted by a UE, etc.

In step S23, the BS generates coded bit stream by performing channelcoding to the information bit stream attached the CRC. The channelcoding scheme is not limited. For example, convolution coding scheme canbe used. The number of PDCCH bits may differ in accordance with channelcoding rate.

In step of S24, the BS generates rate matched bit stream by performingrate matching to the coded bit stream. In step of S25, the BS generatesmodulated symbols by modulating the rate matched bit stream. In step ofS26, the BS maps the modulated symbols to resource elements.

As described above, a method of configuring one PDCCH is explained.However, a plurality of control channels may be transmitted in asubframe. That is, a plurality of PDCCHs for several UEs can betransmitted by being multiplexed in one subframe. Generation ofinformation bit stream, CRC attachment, channel coding, and ratematching, etc. are performed independently for each PDCCH. Theaforementioned process of configuring the PDCCH of FIG. 14 can beperformed independently for each PDCCH.

FIG. 15 shows an example of a method of multiplexing a plurality ofPDCCHs for a plurality of UEs, performed by a BS.

Referring to FIG. 15, a CCE set constituting a control region in asubframe is constructed of a plurality of CCEs indexed from 0 toN(CCE)−1. That is, the number of CCEs is N(CCE). A PDCCH for a UE #1 istransmitted on a CCE aggregation with a CCE index 0 at a CCE aggregationlevel of 1. A PDCCH for a UE #2 is transmitted on a CCE aggregation witha CCE index 1 at a CCE aggregation level of 1. A PDCCH for a UE #3 istransmitted on a CCE aggregation with CCE indices 2 and 3 at a CCEaggregation level of 2. A PDCCH for a UE #4 is transmitted on a CCEaggregation with CCE indices 4, 5, 6, and 7 at a CCE aggregation levelof 4. A PDCCH for a UE #5 is transmitted on a CCE aggregation with CCEindices 8 and 9 at a CCE aggregation level of 2.

The CCEs are mapped to resource elements (REs) according to a CCE-to-REmapping rule. In this case, a PDCCH of each UE is mapped to the REs bybeing interleaved in the control region in the subframe. Locations ofthe REs to be mapped may change according to the number of OFDM symbolsused for transmission of the PDCCHs in the subframe, the number of PHICHgroups, the number of Tx antennas, and the frequency shifts.

The BS does not provide the UE with information indicating where a PDCCHof the UE is located in the subframe. In general, in a state where theUE does not know a location of the PDCCH of the UE in the subframe, theUE finds the PDCCH of the UE by monitoring a set of PDCCH candidates inevery subframe. Monitoring implies that the UE attempts decoding each ofthe PDCCH candidates according to all the monitored DCI formats. This isreferred to as blind decoding or blind detection. If no CRC error isdetected if the UE performs CRC checking after de-masking C-RNTI from aPDCCH candidate, it is regarded that the PDCCH candidate is detected bythe UE as the PDCCH of the UE.

In addition, the UE does not know at which CCE aggregation level thePDCCH of the UE is transmitted. Therefore, the UE needs to attemptdecoding on the set of PDCCH candidates for each of all possible CCEaggregation levels.

FIG. 16 shows an example of a method of monitoring a control channel,performed by a UE.

Referring to FIG. 16, a CCE set constituting a control region in asubframe is constructed of a plurality of CCEs indexed from 0 toN(CCE)−1. That is, the number of CCEs is N(CCE). There are four types ofa CCE aggregation level L, that is, {1, 2, 4, 8}. A set of PDCCHcandidates monitored by the UE is differently defined according to theCCE aggregation level. For example, if the CCE aggregation level is 1,the PDCCH candidates correspond to all CCEs constituting the CCE set. Ifthe CCE aggregation level is 2, the PDCCH candidates correspond to a CCEaggregation with CCE indices 0 and 1, a CCE aggregation with CCE indices2 and 3, and so on. If the CCE aggregation level is 4, the PDCCHcandidates correspond to a CCE aggregation with CCE indices 0 to 3, aCCE aggregation with CCE indices 4 to 7, and so on. If the CCEaggregation level is 8, the PDCCH candidates correspond to a CCEaggregation with CCE indices 0 to 7, and so on.

A frame structure of a single carrier system, a PDCCH transmission andmonitoring method, etc., have been described above. For optimization ofa multi-carrier system, a multi-antenna scheme or a control channelshall be designed by considering a frequency channel property for eachCC. Therefore, it is important to properly use a system parameter and anoptimal transmission/reception scheme for each CC. In addition, the sameframe structure as a legacy system may be used in one CC of themulti-carrier system. In this case, the control channel shall beproperly modified to operate both a UE for the legacy system and a UEfor the multi-carrier system. Hereinafter, the UE for the legacy systemis referred to as a long term evolution (LTE) UE, and the UE for themulti-carrier system is referred to as an LTE-advanced (LTE-A) UE.

FIG. 17 shows an example of a PDCCH transmission method in amulti-carrier system.

Referring to FIG. 17, the multi-carrier system uses a plurality of CCs,i.e., CC #1, CC #2, . . . , CC #L. A PDCCH for a UE #1 is transmittedfor each CC in every subframe. The UE #1 has to attempt blind decodingto find the PDCCH of the UE #1 for each CC in every subframe.

Therefore, if L downlink CCs are used in the multi-carrier system, theLTE-A UE has to receive the PDCCH with a reception complexity which is Ltimes higher than that of the LTE UE. This causes a problem of greatpower consumption in the LTE-A. To solve this problem, there is a needfor an effective control channel transmission method and control channelmonitoring method in the multi-carrier system, whereby a receptioncomplexity of the control channel can be minimized according to ascheduling condition or a channel condition.

FIG. 18 is a flow diagram showing a control channel transmission methodand/or a control channel monitoring method according to an embodiment ofthe present invention.

Referring to FIG. 18, a BS transmits a PDCCH map to a UE (step S110).The UE monitors a set of PDCCH candidates based on the PDCCH map (stepS120).

To decrease a blind decoding complexity of the UE, the PDCCH mapincludes information regarding a PDCCH transmitted by the BS to the UE.The PDCCH map may include a monitoring set field and/or a CCE field. ThePDCCH map may be configured differently in accordance with each DCIformat

First, the monitoring set field is described.

If a UE uses L downlink CCs, a BS can transmit a PDCCH simultaneouslythrough N downlink CCs out of the L downlink CCs (L≧N, where L and N arenatural numbers). In this case, when the BS informs the UE of the Ndownlink CCs on which the PDCCH is transmitted, the UE performs blinddecoding only in the N downlink CCs. Accordingly, a blind decodingcomplexity of the UE can be decreased. That is, the monitoring set fieldindicates the N downlink CCs out of the L downlink CCs. The BS transmitsthe PDCCH to the UE only through the N downlink CCs. The UE monitors thePDCCH only in the N downlink CCs. That is, the UE monitors a set ofPDCCH candidates in each of the N downlink CCs. The monitoring set fieldmay be configured differently in accordance with each DCI format.

FIG. 19 shows an example of transmitting a PDCCH by using a PDCCH map ina multi-carrier system.

Referring to FIG. 19, the PDCCH map includes a monitoring set field. ThePDCCH map is transmitted only through a CC #1. The PDCCH map isdynamically transmitted in every subframe. If the PDCCH map isdynamically transmitted, flexibility of scheduling can be increased. ThePDCCH map transmission method of FIG. 19 is for exemplary purposes only,and the PDCCH map transmission method of the present invention is notlimited thereto.

Hereinafter, a radio resource for transmitting a PDCCH map will bedescribed. The radio resource used for PDCCH map transmission may beconstructed by combining a time resource, a frequency resource, and/or acode resource. The radio resource by which the PDCCH map is transmittedmay be determined according to a rule predetermined between a BS and aUE. Alternatively, the PDCCH map may be transmitted in a PDCCH format.That is, in a state where the UE does not know a location of the PDCCHmap in a subframe, the UE can attempt blind decoding to find a PDCCH mapin every subframe. For example, the PDCCH map may be generated in such amanner that an information bit stream based on the PDCCH map format isgenerated and then an identifier (ID) of the UE, which is an owner ofthe PDCCH map, is masked to a CRC. The PDCCH map format may include amonitoring set field and/or a CCE field as an information field.

Even if the PDCCH map is transmitted in the PDCCH format, it ispreferable that the UE knows a downlink CC on which the PDCCH map istransmitted. This is because a purpose of transmitting the PDCCH map isto reduce the blind decoding complexity, and this purpose is notachieved if the UE does not know the downlink CC on which the PDCCH mapis transmitted.

Hereinafter, a CC on which a PDCCH map is transmitted will be described.

A BS can transmit the PDCCH map to a UE through a constant downlink CC.Alternatively, the downlink CC on which the PDCCH map is transmitted maychange over time. For example, the downlink CC on which the PDCCH map istransmitted may change with a specific period according to a channelcondition. Alternatively, the downlink CC on which the PDCCH map istransmitted may change in a specific pattern according to a hopping rulepredetermined between the BS and the UE. In this case, the PDCCH map canbe transmitted by being distributed over a plurality of downlink CCs. Inanother method, the BS may configure a downlink CC on which the PDCCHmap is transmitted semi-statically through higher layer signaling suchas a radio resource control (RRC) signaling. In this case, the downlinkCC on which the PDCCH map is transmitted is semi-statically modified.

The downlink CC on which the PDCCH map is transmitted may be determinedaccording to the UE. A plurality of downlink CCs to be allocated maydiffer according to the UE. The UE can use a downlink CC of a lowestfrequency band among the plurality of allocated downlink CCs intransmission of the PDCCH map. This is because a low frequency band hasa high reliability.

If the PDCCH is not transmitted in all downlink CCs, the BS may transmitthe PDCCH map to the UE to report a presence or absence of the PDCCH.Alternatively, the BS may not transmit the PDCCH map to the UE, whichcan be regarded as error occurrence.

FIG. 20 shows another example of transmitting a PDCCH by using a PDCCHmap in a multi-carrier system.

Referring to FIG. 20, the multi-carrier system uses three downlink CCs,i.e., CC #1, CC #2, and CC #3. The PDCCH map includes a monitoring setfield. A PDCCH map of a UE #1 is transmitted through the CC #1 in everysubframe. In a subframe n, a monitoring set field for the UE #1indicates the CC #1, the CC #2, and the CC #3. Therefore, the PDCCHs forthe UE #1 are transmitted through each of the CC #1, the CC #2, and theCC #3. In a subframe n+1, the monitoring set field for the UE #1indicates the CC #2. Therefore, the PDCCH for the UE #1 is transmittedonly through the CC #2. In a subframe n+k, the monitoring set field forthe UE #1 indicates the CC #1 and the CC #3. Therefore, the PDCCHs forthe UE #1 are transmitted through each of the CC #1 and the CC #3.

The monitoring set field may use a bitmap to indicate a downlink CC onwhich a PDCCH is transmitted. A plurality of downlink CCs correspond torespective bits of the monitoring set field, and the downlink CC onwhich the PDCCH is transmitted may be expressed by ‘1’. For example, incase of FIG. 20, the monitoring set field may have a size of 3 bits. Inthe subframe n, the monitoring set field for the UE #1 may be 111. Inthe subframe n+1, the monitoring set field for the UE #1 may be 010. Inthe subframe n+k, the monitoring set field for the UE #1 may be 101.

In FIG. 19 and FIG. 20, the PDCCH map is dynamically transmitted inevery subframe. In this case, the downlink CC on which the PDCCH istransmitted changes dynamically in every subframe.

The BS may configure the PDCCH map semi-statically via higher layersignaling such as RRC. In this case, the downlink CC on which the PDCCHis transmitted changes semi-statically.

FIG. 21 shows an example of semi-statically configured a PDCCH map.

Referring to FIG. 21, a multi-carrier system uses three downlink CCs,i.e., CC #1, CC #2, and CC #3. The PDCCH map includes a monitoring setfield. It is assumed that a monitoring field set for the UE #1 indicatesthe CC #1 and the CC #3 and does not indicate the CC #2. The PDCCH mapis semi-statically configured, and thus the PDCCH for the UE #1 istransmitted only through the CC #1 and the CC #3 from a subframe n to asubframe n+k. In FIG. 21, a PDSCH corresponding to the PDCCH istransmitted only through a downlink CC on which the PDCCH istransmitted. That is, if the PDCCH is transmitted through the CC #1, thePDSCH corresponding to the PDCCH is transmitted only through the CC #1.In this case, the PDSCH for the UE #1 cannot be transmitted through theCC #2. Therefore, the UE #1 may not receive any information in the CC#2. If the BS intends to transmit small-sized downlink data to the UE #1through the PDSCH, there is no problem even if the CC #2 is not used fortransmission of downlink data for the UE #1.

If a radio resource scheduling scheme is a semi-persistent scheduling(SPS) scheme, the UE can read downlink data on the PDSCH without havingto receive the PDCCH. Therefore, the BS can transmit the PDSCH based onthe SPS scheme through the CC #2.

FIG. 22 shows another example of semi-statically configured a PDCCH map.In FIG. 22, similarly to FIG. 21, a multi-carrier system uses threedownlink CCs, i.e., CC #1, CC #2, and CC #3. It is assumed that amonitoring field set for the UE #1 is included in the PDCCH map, andindicates the CC #1 and the CC #3 and does not indicate the CC #2.

Referring to FIG. 22, a downlink CC on which a PDSCH is transmitted maybe equal to or different from a downlink CC on which a PDCCH forscheduling the PDSCH is transmitted. Therefore, a PDCCH for the UE #1 istransmitted only through the CC #1 and the CC #3 indicated by themonitoring set field, whereas the PDSCH may be transmitted through alldownlink CCs, i.e., the CC #1 to the CC #3. In a subframe n+1, a PDCCHfor the CC #2 is transmitted through the CC #1. In this case, accordingto a CC indicator or a predetermined rule, the UE #1 can know for whichdownlink CC the PDCCH is used.

If L CCs can be constructed for one LTE-A UE, only A CCs out of the LCCs can be used (L≧N, where L and N are natural numbers). An active CCset is a CC set having the A CCs out of the L CCs as its elements. Theaforementioned monitoring set field can be used in the active CC setdefined with the A CCs.

Next, the CCE field is described.

A UE has to attempt blind decoding for each CCE aggregation level.Therefore, a blind decoding complexity can be significantly decreased ifthe CCE aggregation level to be monitored by the UE is limited. If awireless communication system uses X CCE aggregation levels, a BS cantransmit a PDCCH to the UE by using Y CCE aggregation levels (X≧Y, whereX and Y are natural numbers). In this case, when the BS reportsinformation regarding the Y CCE aggregation levels to the UE, the UEperforms blind decoding only for the Y CCE aggregation levels.Accordingly, the blind decoding complexity of the UE can be decreased.That is, the CCE field indicates the Y CCE aggregation levels out of theX CCE aggregation levels. The UE can monitor a PDCCH only for each ofthe Y CCE aggregation levels indicated by the CCE field. The Y CCEaggregation levels indicated by the CCE field will be referredhereinafter as a subset level.

For example, if the wireless communication system uses 4 CCE aggregationlevels, such as, {1, 2, 4, 8}, the subset level can be indicatedaccording to a CCE field value as described in the following table.

TABLE 4 CCE field Subset level 0 {1, 2, 4, 8} 1 {1, 2, 4} 2 {2, 4, 8} 3{4, 8}

Table 4 is for exemplary purposes only, and thus the subset level can bevariously configured according to the CCE field value. The CCE field mayindicate a different subset level for each a DCI format or a DCI formatgroup. Alternatively, the CCE field may indicate a subset level only fora specific DCI format by using the CCE field. In addition, the CCE fieldmay indicate a different subset level for each a downlink CC or adownlink CC group.

The CCE field may be included in a PDCCH map. Therefore, theaforementioned description on the PDCCH map can be equally applied tothe CCE field. The CCE field can be transmitted dynamically orsemi-statically.

As such, the CCE aggregation level may be explicitly limited to thesubset level by using the CCE field. Alternatively, the CCE aggregationlevel may be implicitly limited to the subset level. For example, allPDCCHs in a subframe may be specified to use the same CCE aggregationlevel. If the UE finds one PDCCH from a plurality of PDCCHs at one CCEaggregation level, the UE monitors the remaining PDCCHs also at the CCEaggregation level.

FIG. 23 shows a control channel monitoring method performed by a UE in amulti-carrier system.

Referring to FIG. 23, three PDCCHs, i.e., PDCCH #1, PDCCH #2, and PDCCH#3, for a UE #1 are allocated to logically contiguous CCEs on a CCEaggregation.

As such, a BS can transmit a plurality of PDCCHs for one UE through onedownlink CC. The plurality of PDCCHs can have independent CCEaggregation levels. The plurality of PDCCHs can be allocated tologically contiguous CCEs. If the UE detects the PDCCH #2, a CCE index(4 to 7) of a CCE aggregation on which the PDCCH #2 is transmitted canbe used to facilitate detection of another PDCCH from CCEs located in afront or rear portion in the CCE aggregation.

FIG. 24 is a block diagram showing wireless communication system toimplement an embodiment of the present invention. A BS 50 may include aprocessor 51, a memory 52 and a radio frequency (RF) unit 53. Theprocessor 51 may be configured to implement proposed functions,procedures and/or methods described in this description. Layers of theradio interface protocol may be implemented in the processor 51. Thememory 52 is operatively coupled with the processor 51 and stores avariety of information to operate the processor 51. The RF unit 53 isoperatively coupled with the processor 11, and transmits and/or receivesa radio signal. A UE 60 may include a processor 61, a memory 62 and a RFunit 63. The processor 61 may be configured to implement proposedfunctions, procedures and/or methods described in this description. Thememory 62 is operatively coupled with the processor 61 and stores avariety of information to operate the processor 61. The RF unit 63 isoperatively coupled with the processor 61, and transmits and/or receivesa radio signal.

The processors 51, 61 may include application-specific integratedcircuit (ASIC), other chipset, logic circuit, data processing deviceand/or converter which converts a baseband signal into a radio signaland vice versa. The memories 52, 62 may include read-only memory (ROM),random access memory (RAM), flash memory, memory card, storage mediumand/or other storage device. The RF units 53, 63 include one or moreantennas which transmit and/or receive a radio signal. When theembodiments are implemented in software, the techniques described hereincan be implemented with modules (e.g., procedures, functions, and so on)that perform the functions described herein. The modules can be storedin memories 52, 62 and executed by processors 51, 61. The memories 52,62 can be implemented within the processors 51, 61 or external to theprocessors 51, 61 in which case those can be communicatively coupled tothe processors 51, 61 via various means as is known in the art.

Accordingly, a reception complexity of a PDCCH in the UE can bedecreased in a multi-carrier system. A method and an apparatus ofeffectively monitoring a PDCCH are provided. As a result, powerconsumption of the UE can be decreased. Therefore, overall systemperformance can be improved.

In view of the exemplary systems described herein, methodologies thatmay be implemented in accordance with the disclosed subject matter havebeen described with reference to several flow diagrams. While forpurposed of simplicity, the methodologies are shown and described as aseries of steps or blocks, it is to be understood and appreciated thatthe claimed subject matter is not limited by the order of the steps orblocks, as some steps may occur in different orders or concurrently withother steps from what is depicted and described herein. Moreover, oneskilled in the art would understand that the steps illustrated in theflow diagram are not exclusive and other steps may be included or one ormore of the steps in the example flow diagram may be deleted withoutaffecting the scope and spirit of the present disclosure.

What has been described above includes examples of the various aspects.It is, of course, not possible to describe every conceivable combinationof components or methodologies for purposes of describing the variousaspects, but one of ordinary skill in the art may recognize that manyfurther combinations and permutations are possible. Accordingly, thesubject specification is intended to embrace all such alternations,modifications and variations that fall within the spirit and scope ofthe appended claims.

1-10. (canceled)
 11. A method of monitoring a physical downlink controlchannel (PDCCH) in a wireless communication system, carried in a userequipment (UE), the method comprising: receiving, from a base station(BS), a PDCCH map comprising a monitoring set field indicating amonitoring set comprising N downlink (DL) component carriers (CCs) amongL DL CCs, where L≧N, L and N each is a natural number; and monitoring aset of PDCCH candidates at each of the N DL CCs.
 12. The method of claim11, wherein the step of monitoring a set of PDCCH candidates includes:decoding each PDCCH candidate; and if a PDCCH candidate is successfullydecoded, acquiring control information on the successfully decoded PDCCHcandidate.
 13. The method of claim 12, wherein the PDCCH candidate issuccessfully decoded if no CRC error is detected after de-masking theCRC of the PDCCH candidate with a UE's identifier.
 14. The method ofclaim 13, wherein the PDCCH map further comprises a CCE field, the CCEfield indicating the Y CCE aggregation levels among X CCE aggregationlevels, where X≧Y, X and Y each is a natural number.
 15. The method ofclaim 14, wherein the UE monitors the set of PDCCH candidates at each ofY control channel element (CCE) aggregation levels.
 16. The method ofclaim 11, wherein the PDCCH map is received on a PDCCH.
 17. The methodof claim 11, wherein the PDCCH map is received through a specific DL CCamong the L DL CCs.
 18. The method of claim 11, wherein the PDCCH map isreceived through a DL CC, the DL CC being changed among the L DL CCs inaccordance with a predefined rule.
 19. The method of claim 11, whereinthe PDCCH map is received via a radio resource control (RRC) message.20. A UE comprising: radio frequency (RF) unit transmitting andreceiving a radio signal; and a processor coupled with the RF unit andconfigured to receive, from a base station (BS), a PDCCH map comprisinga monitoring set field indicating a monitoring set comprising N downlink(DL) component carriers (CCs) among L DL CCs, where L≧N, L and N each isa natural number, and to monitor a set of PDCCH candidates at each ofthe N DL CCs.
 21. The UE of claim 20, wherein the processor is furtherconfigured to; decode each PDCCH candidate; and if a PDCCH candidate issuccessfully decoded, acquire control information on the successfullydecoded PDCCH candidate.
 22. The UE of claim 21, wherein the PDCCHcandidate is successfully decoded if no CRC error is detected afterde-masking the CRC of the PDCCH candidate with the UE's identifier. 23.The UE of claim 22, wherein the PDCCH map further comprises a CCE field,the CCE field indicating the Y CCE aggregation levels among X CCEaggregation levels, where X≧Y, X and Y each is a natural number.
 24. TheUE of claim 23, wherein the processor monitors the set of PDCCHcandidates at each of the Y control channel element (CCE) aggregationlevels.
 25. The UE of claim 20, wherein the PDCCH map is received on aPDCCH.
 26. The UE of claim 20, wherein the PDCCH map is received througha specific DL CC among the L DL CCs.
 27. The UE of claim 20, wherein thePDCCH map is received through a DL CC, the DL CC being changed among theL DL CCs in accordance with a predefined rule.
 28. The UE of claim 20,wherein the PDCCH map is received via a radio resource control (RRC)message.